# How can I tell from the location of flow separation if an airfoil is stalled?

I am in the process of completing a carbon airfoil for my track car (sedan) and I am looking to determine stall angle visually. AKA, wool tufts and highway runs with my GoPro pointed upward from the trunklid.

Since I don't really know the angle of incidence at which the air coming off the rear window will hit the wing, visually looking for flow separation is really the only way for me to determine my max angle and work down from there based on the car's balance at the track.

Is there a paper (or common knowledge) regarding over what % of chord length flow separation must occur for the wing to be considered stalled? Highly cambered airfoil, Selig S1223, 11.5" chord, 66" span. I have searched for papers on the topic but cannot find anything.

Thanks!

• Is there any way for you to measure the amount of downforce you're getting? When the airfoil stalls that should decrease significantly Mar 28, 2019 at 18:50
• Yes, but that would be more involved in terms of adding suspension components than I really want to get. Mar 28, 2019 at 19:47

## 2 Answers

The boundary layer separates usually at somewhere around 10-15% of chord near stall AOA (which is where you place vortex generators when using them to delay stall AOA). Separation will start at the trailing edge and move forward. If it was me I'd tuft the entire lower surface from just behind the LE, maybe 10% so you can really see everything that's going on. Doesn't hurt to put too many on.

The location of separation is very hard to predict accurately and the range can be quite large when comparing leading-edge and trailing-edge stall. So tuffing the entire lower surface is indeed recommended.

XFoil can get you a good initial guess though.

The stall behavior can differ significantly at different Reynolds numbers (read speed). In order to be able to map you video to GPS data, you might want to include a clock in your image, so you can limit the amount of test runs.

As a sidenote, I assume you're also considering a Gurney Flap?